In a large pot, prepare macaroni and cheese according to package directions; add cooked hamburger.
Slice bacon into approx. 1/2-inch pieces and fry until crisp. Drain and reserve 2 tablespoons bacon fat, saving any remaining bits. Add bacon and 1 tablespoon fat to pot with macaroni and hamburger mixture. Add diced onion and remaining 1 tablespoon bacon fat to pan and brown until edges are brown and crispy.
Using a rubber spatula, scrape cooked onion and remaining bacon fat into macaroni and cheese mixture. Add tomato soup, stir and heat thoroughly. Serve with warm, buttered bread.